Microwave dielectric ceramic compositions

ABSTRACT

A microwave dielectric ceramic composition consisting of 15-19 mol % of barium oxide (BaO), 61-73 mol % of titanium dioxide (TiO 2 ), 3.5-15 mol % of samarium oxide (Sm 2  O 3 ), and 1.5-15 mol % of cerium oxide (CeO 2 ). Since this composition has a high specific dielectric constant ε r  and a high no load value Q, it can be used as a microwave resonator, a temperature compensating capacitor or the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a dielectric ceramic composition for use in making microwave devices, and more particularly to a microwave dielectric ceramic composition which exhibits a high dielectric constant ε_(r) and a high no load Q value, and, in addition, a positive or negative temperature coefficient η_(f) of about zero can be obtained by changing the ceramic composition.

2. Prior Art

Ceramic capacitors utilized for temperature compensation, as well as dielectric resonators for use in microwave circuits or the like have been required to have a high specific dielectric constant ε_(r) as well as a large no load value Q and further should obtain a positive or negative temperature coefficient η_(f) of the resonance frequency of about zero in consideration of the temperature coefficient of metal to be used in conjunction with the dielectric ceramic composition.

As such a dielectric ceramic composition, a BaO-TiO₂ type, a MgO-TiO₂ -CaO type, and a ZrO₂ -SnO₂ -TiO₂ type composition and the like have heretofore been employed. However, when a dielectric resonator or a capacitor is fabricated with these ceramic compositions, their specific dielectric constant ε_(r) becomes a low value in the order of 20-40 in the vicinity where its temperature coefficient η_(f) is zero (ppm/°C.). Consequently, the size of such an oversized construction of apparatuses as dielectric resonators and the like fabricated with these materials become too large.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an improved dielectric ceramic composition for use in microwave circuits capable of manifesting a high specific dielectric constant ε_(r) and a high no load Q near zero (ppm/°C.) of the temperature coefficient η_(f).

According to this invention there is provided a microwave ceramic composition for use in microwave circuits, comprising 15-19 mol% of BaO, 61-73 mol% of TiO₂, 3.5-15 mol% of Sm₂ O₃ and 1.5-15 mol% of CeO₂.

PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the invention will be described as follows.

As starting materials, high purity BaCO₃, TiO₂, Sm₂ O₃ and CeO₂ were weighed in accordance with the predetermined compositional ratios shown in the following Tables 1 and 2 and were mixed together with pure water in a pot mill. The mixture was dehydrated and dried. The resulting mixture was calcined at 1060° C. for 2 hours in air. The calcined product thus obtained was subjected to wet pulverization together in pure water in a pot mill. The pulverized slurry was dehydrated and dried to obtain a powder. To the powder was added a binder to obtain a granulated product, which was classified by passing the product through a 32 mesh sieve. The granulated product was molded into a circular disk having a diameter of 16 mm and a thickness of 9 mm by means of a metal mold and an oil press under a molding pressure of 1-3 tons/cm². The molded product was placed in a casing made of high purity alumina, and fired at a temperature of 1260°-1500° C. for 2 hours to produce a dielectric ceramic composition. The specific dielectric constant ε_(r) and no load value Q of the resulting ceramic composition was measured in accordance with the Hakki-Coleman method. Furthermore, the temperature coefficient η_(f) of the resonance frequency was determined from the values in the temperature range of -40° C. to +80° C. on the basis of the resonance frequency at 20° C. according to the following equation (1): ##EQU1## wherein f(20): resonance frequency at 20° C.,

f(-40): resonance frequency at -40° C.,

f(80): resonance frequency at 80° C., and

ΔT: temperature difference, i.e. 80-(-40)=120° C. in this case.

In the above measurement, the resonance frequency was 3-7 GHz. The results of the experiments are summarized in Table 2.

                  TABLE 1                                                          ______________________________________                                                          Range of Composition                                          Composition      (mol %)                                                       ______________________________________                                         BaO.(TiO.sub.2).sub.x                                                                            70-95                                                        wherein x = 3.7-4.3                                                            Sm.sub.2 O.sub.3 3.5-15                                                        CeO.sub.2        1.5-15                                                        ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                                                                    η.sub.f                         Sample                                                                               (BaO) · (TiO.sub.2).sub.x                                                         Sm.sub.2 O.sub.3                                                                        CeO.sub.2       (ppm/                               No.   (mol %)     (mol %)  (mol %)                                                                               ε.sub.r                                                                    Q    °C.)                         ______________________________________                                          1*   91          1.8      7.2    40  1500 +130                                2     "           3.5      5.5    55  2500 +20                                 3     "           7.5      1.5    57  2600 -15                                 4     83.4        8.3      8.3    66  2550  +9                                 5     "           9.8      6.8    67  2000   0                                 6     "           13.3     3.3    68  1800 -20                                 7     76.9        8.1      15.0   65  1750 +15                                 8     "           9.2      13.9   66  1700  +8                                 9     "           15.0     8.1    70  1600 +15                                 10    70          15.0     15.0   65  1500 +20                                 11    95          3.5      1.5    50  2800   0                                 12*   83.5        16.5     0      64  1500 -80                                 13*   97          1        2      33  2700 +20                                 14*   65          12.5     22.5   35   700 +120                                15*   84.5        14.5     1.0    66  1200 -70                                 ______________________________________                                          *Control Examples                                                        

According to the results shown in the above Tables, it has been found that there are such problems that temperature coefficient η_(f) becomes high and that the dielectric constant ε_(r) becomes small, i.e., 40 or less in the case where the amount of BaO.(TiO₂)_(x) has a value less than 70 mol%, and the amount of Sm₂ O₃ is less than 3.5 mol%, or the amount of CeO₂ exceeds 15 mol%.

Furthermore, when BaO.(TiO₂)_(x) exceeds 95 mol%, the dielectric constant ε_(r) becomes low. In addition, the temperature coefficient η_(f) becomes too high when Sm₂ O₃ exceeds 15 mol%, or CeO₂ is less than 1.5 mol%.

Thus, such a range that Bao: 15-19 mol%, TiO₂ : 61-73 mol%, Sm₂ O₃ : 3.5-15 mol%, and CeO₂ : 3.5-15 mol% is most suitable as a dielectric ceramic composition for microwave circuits from a practical point of view.

As is apparent from the description given above, the dielectric ceramic composition according to this invention exhibits a high specific dielectric constant ε_(r) and a high no load value Q in the microwave region. Furthermore, since the temperature coefficient η_(f) can be controlled to cover a wide range by changing its composition, the composition of this invention can be efficiently utilized for microwave dielectric resonators, as capacitors for temperature compensation or the like so that the industrial usefulness of the dielectric ceramic composition in accordance with the present invention is remarkably high. 

What is claimed is:
 1. A microwave dielectric ceramic composition consisting of 15-19 mol% of barium oxide (BaO), 61-73 mol% of titanium dioxide (TiO₂), 3.5-15 mol% of samarium oxide (Sm₂ O₃), and 1.5-15 mol% of cerium oxide (CeO₂). 